1. Field of the Invention
The present invention relates to an optical communication system, and more particularly, to an optical module, which is used to receive, transmit, or repeat an optical signal in a high frequency band, and an optical module system in which the optical module is integrated.
2. Description of the Related Art
In order to realize an optical module, elements necessary for realizing optical communications are integrated on a substrate. Such an optical module includes an optical receiver module used to receive an optical signal, an optical transmitter module used to transmit an optical signal, an optical repeater module used to repeat an optical signal, or the like. In particular, the optical receiver module receives an optical signal from an optical communication system, converts the optical signal into an electrical signal, and transmits the electrical signal to the communication system.
A conventional optical module, particularly an optical receiver module, is constituted by integrating various types of elements on a silicon substrate. However, within an increase in a communication frequency used in an optical communication, the use of such a silicon substrate reaches the limit. The reason is that the silicon substrate has a semiconducting characteristic, which causes an increase in loss of a signal.
Accordingly, in a high frequency, low-power consuming optical communication system, the use of an optical module employing such a silicon substrate is limited. An optical receiver module is realized by using a compound semiconductor substrate, such as GaAs, having less signal loss in a high frequency band, or a semiconductor substrate, such as SOI or SOS, instead of such a silicon substrate. SOI or SOS is developed to reduce signal loss by interposing an additional insulating layer between a substrate and elements integrated on the substrate to reduce the effect of the semiconductivity of silicon. However, since these substrates are much expensive than a silicon substrate, the realization of an optical module using these substrates has a high cost.
In actual, even when a silicon substrate is used, a silicon oxide (SiO2) is interposed between the silicon substrate and elements integrated on the silicon substrate. Since the silicon oxide has a low dielectric constant, the silicon oxide is suitable for being used in an optical module in a high frequency band. Nevertheless, the use of the silicon substrate in the optical module in a high frequency band is restricted since the silicon oxide is formed to a thickness much thinner than a desired thickness on the silicon substrate.
In order to reduce signal loss in a high frequency band, the silicon oxide has to have a thickness of about several tens of micrometers. However, the thickness of an oxide layer grown on the silicon substrate by oxidation does not come to the desired thickness.
In general, a silicon oxide layer is formed on a silicon substrate by penetrating oxygen into the surface of the silicon substrate, i.e., performing an oxidation process, and converting a silicon layer into a silicon oxide layer. However, as the thickness of the silicon oxide layer increases with the growth thereof, oxygen cannot further penetrate into silicon. In other words, oxygen is trapped in the silicon oxide layer and does not penetrate into silicon under the silicon oxide layer any more. Due to this, when the thickness of the silicon oxide layer reaches 10 μm, it comes to a saturated point. Thus, the thickness of the silicon oxide layer is not increased any more.
As described above, it is difficult to thickly form a silicon oxide layer on a silicon substrate. Thus, it is difficult to employ the silicon substrate in an optical module using light in a high frequency band to realize the optical module.